The present invention is generally related to an improved loudspeaker assembly for frequencies in a range of approximately 150 Hz to 6 kHz.
The basic theory of sound transmission is set forth, for example, in standard introductory physics textbooks such as Resnick and Halliday, Physics, Part I, John Wiley & Sons, 1977, pp. 433-456. As described therein, audible sound is a longitudinal mechanical wave having a frequency within a range of approximately 20 Hz to 20 kHz. Typically, sound is generated by vibrating elements which alternately compress the surrounding air on a forward movement and rarefy it on a backward movement. Air transmits these disturbances outward from the source as a longitudinal wave. Upon entering the ear, these waves produce the sensation of sound.
A loudspeaker is generally understood to be a device which converts electrical energy into sound energy. Different loudspeakers are utilized to reproduce various parts of the audible frequency range. A woofer is generally responsive only to the lower acoustic frequencies and reproduces sounds of low pitch. A tweeter, in contrast, is a relatively small loudspeaker responsive only to the higher acoustic frequencies and reproduces sounds of high pitch. A so-called mid-range speaker reproduces sounds having frequencies intermediate of the woofer and tweeter.
The primary components of a loudspeaker are an electromagnet and a vibrating diaphragm attached to an armature that is vibrated by the variations of electric current in the electromagnet. A cone speaker is a particular type of loudspeaker in which the vibrating diaphragm is relatively large and conical and usually made of paper. A simple cone speaker assembly 3 is shown in FIG. 1 and includes a speaker housing or cabinet 5, transducer 6, and speaker cone 7. Transducer 6 causes speaker cone 7 to vibrate in response to signals from an amplifier (not shown), thus producing sound in the manner described above. The vibration of speaker cone 7 generates two longitudinal sound waves which, at least initially, propagate in opposite directions. Front waves 8 propagate to the right in FIG. 1 while back waves 9 propagate to the left. It is generally this front wave which generates the sounds, such as music, heard by a listener. The sounds heard by a listener are often directional in nature, i.e., they are dependent on the relative positioning of the loudspeaker and the listener. Thus, the loudspeakers in a room must be carefully arranged by a listener to attempt to properly direct the sound for maximum acoustic quality. However, even careful arrangement of speakers within a room is often unsatisfactory since it is unlikely that all listeners in a given room will be positioned so as to be in a region of maximum acoustic quality.
It will be appreciated that as speaker cone 7 is moved to the right, a compression is generated in the front wave while a rarefaction is generated in the back wave. Similarly, when speaker cone 7 is moved to the left, a rarefaction is generated in the front wave and a compression is generated in the back wave. Thus, the front wave and back wave are 180.degree. out of phase. If the back waves are permitted to emanate from speaker housing 6 at the same time as or "coincident" with the front waves, the waves will tend to destructively interfere or cancel since they are out of phase. This results in low quality sound reproduction by the loudspeaker. However, the elimination or non-use of this back wave in conventional speakers is inherently inefficient, i.e., half of the sound reproducing capabilities of the speaker are not utilized.
The vibration of speaker cone 7 generates subordinate vibrations such as cabinet vibrations. This is particularly true of cabinets formed of acoustically active materials such as wood which are relatively sensitive to vibratory forces. These subordinate vibrations modify or "cloud" the sound generated by the excitation of speaker cone 7. This effect is known as intermodulation (IM) distortion. The intermodulation produces resultant waves with frequencies that are equal to the sums and differences of integral multiples of the cone vibration frequency and the cabinet vibration frequency. Other subordinate vibrations also contribute to the IM distortion. IM distortion is an important factor which must be addressed in speaker design. Conventional speakers have IM distortion of 10-20%.
Since loudspeakers are designed to reproduce sound as faithfully as possible, the design must be compatible with the physics of sound, particularly if it is desired to reproduce high fidelity musical sound. Musical waveforms have a sinusoidal basis. Since sine waves are algebraic, a sound reproduction device should be curvilinear, rather than rectilinear, in design. An example of such design is the "bell-type" shape of many musical instruments.
All musical sound waves travelling in a medium (such as air, water, etc.) are acoustical (or physical). The sound waves are also algebraic functions since the fundamental frequency is sinusoidal (as on the lowest notes from a flute). All harmonics, or overtones, are also sinusoidal. Since the function of a loudspeaker is to receive the electronic format of the sound and reproduce the sound as acoustical (or physical) wave action in the medium through which it travels, it follows that the shapes involved in the loudspeaker baffle, resonating and transferral devices should be compatible with this sinusoidal nature. Therefore, the most compatible shapes should be curves, spheres, triangles or pyramids, rather than straight lines, cubes, squares or rectangles. The compatible sinusoidal shapes greatly enhance the sinusoidal characteristics of each fundamental frequency and its complement of harmonics, or overtones. This also applies to "ports" which transfer acoustical sound pressure waves within the speaker assembly, and also to the transfer of the waves to the surrounding transmitting medium.
The present invention is concerned with an improved mid-range speaker which reproduces sounds having a frequency between approximately 150 Hz and 6 kHz.